Regulation of proliferation in sensory ganglia of chick embryos by Nerve Growth Factor (NGF). This controversial issue can be resolved by neuron counts of normal embryos and embryos which have received daily injections of NGF from 4 1/2days on. We plan a) counts of small dorso-medial (DM) neurons in thoracic ganglion 18 at 8 days, that is, after termination of proliferation and before onset of degeneration; b) counts of large ventro-lateral (VL) neurons in brachial ganglion 15 at 8 days. Combination of early wing bud extirpation and daily injections of NGF. To answer the question of whether NGF can alleviate the massive degeneration of sensory neurons resulting from the absence of the target area. No data are available concerning neuronal death in a sympathetic chain ganglia. Counts of degenerating cells will be made from day 5 to posthatching stages in ganglia 15 and 18.